Figure 4: (A-B) Histological characteristics of the trachea (Tr) of the equine embryo at 24 days. (A) Note the absence of invagination of the cartilaginous tissue which will form the tracheal rings. (B) Observe the respiratory epithelium (RE), lamina propria (LP), smooth muscle (SM), vascularized connective tissue (arrows) containing glands and still-forming seromucous cells, and the dense tissue of the perichondrium (Pe) enveloping the hyaline cartilage (Ca). (C-G) Panoramic view of the trachea (Tr) of the equine embryo at 73 days and its syntopy with the esophagus (Es). Note the respiratory epithelium (RE), lamina propria (LP), vascularized connective tissue (arrow), dense connective tissue of the perichondrium (Pe) involving the hyaline cartilage (Ca), smooth muscle (SM), and basal membrane (BM) under the respiratory epithelium (RE).